Enhanced microfluidic electromagnetic measurements

ABSTRACT

Techniques for enhanced microfluidic impedance spectroscopy include causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid. Flow in the channel is laminar. A dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid. Electrical impedance is measured in the channel between at least a first pair of electrodes. In some embodiments, enhanced optical measurements include causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid. An optical index of refraction of a fluid constituting either sheath flow is much less than an optical index of refraction of the core fluid. An optical property is measured in the channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Provisional Appln. 61/388,916, filedOct. 1, 2010, the entire contents of which are hereby incorporated byreference as if fully set forth herein, under 35 U.S.C. §119(e).

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with Government support under CooperativeAgreement No. NNA04CC32A awarded by the National Aeronautics and SpaceAdministration, and Contract #: 1R21 1EB007390-01A1 awarded by theNational Institutes of Health/National Institute of Biomedical Imagingand Bioengineering. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

An impedance-based microfluidic flow cytometer uses a small channel incombination with control of the flow of fluid streams in that channel toguide cells or other particles between two or more electrodes thatmeasure electrical impedance in the vicinity of each electrode orbetween pairs of electrodes. This principle has been applied in variousembodiments, including several published micro flow cytometers. Notethat the term flow cytometer is used loosely herein, as the apparatuscan be used for the characterization of the impedance of a large varietyof particles, not only cells.

SUMMARY OF THE INVENTION

Techniques are provided for enhanced electromagnetic (EM) measurementsin low-shear laminar flows in microfluidic channels. Such techniquesallow precise electrical impedance measurements of delicate orforce-sensitive particles.

According to a first set of embodiments, a method includes causing acore fluid to flow into a channel between two sheath flows of one ormore sheath fluids different from the core fluid. Flow in the channel islaminar. A dielectric constant of a fluid constituting either sheathflow is much less than a dielectric constant of the core fluid. Themethod further comprises determining impedance in the channel between atleast a first pair of electrodes. In some of these embodiments, activefeedback is used to control the width of the flow of the core fluid. Forexample, in some embodiments, a method includes controlling relativepressure or flow rate of a source of the core fluid compared to pressureor flow rate of a source of one or more of the sheath fluids bycontrolling the relative pressure or flow rate to stabilize ameasurement of a property of the core fluid. In some embodiments, theproperty of the core fluid includes position or shape of the core flowin the channel, or both. In some embodiments, the position or shape ofthe core flow, or both, is stabilized based on impedance or opticalmeasurements, e.g., to maintain a certain width or cross-section orposition in the center of the channel or some combination. In someembodiments, the position or shape of the core flow, or both, isstabilized based on measurements of known particles included in the coreflow.

In another set of embodiments, a method comprises causing a core fluidto flow into a channel between two sheath flows of one or more sheathfluids different from the core fluid. An optical index of refraction ofa fluid constituting either sheath flow is much less than an opticalindex of refraction of the core fluid. The method further comprisesmeasuring an optical property in the channel between an optical sourceand an optical detector.

In another set of embodiments, a method comprises causing a core fluidto flow into a channel between two sheath flows of one or more sheathfluids different from the core fluid. A value of a first electromagneticproperty of a fluid constituting either sheath flow is substantiallydifferent from a value of the first electromagnetic property of the corefluid. Flow in the channel is laminar. The method also comprisesmeasuring a second electromagnetic property in the channel using anelectromagnetic signal that is concentrated in the core fluid by adifference in the value of the first electromagnetic property of eithersheath flow and the value of the first electromagnetic property of thecore fluid.

According to various other sets of embodiments, an apparatus comprisesmeans to perform each step of one of the above methods; or acomputer-readable storage medium is configured to cause an apparatus toperform one or more steps of one of the above methods.

Still other aspects, features, and advantages of the invention arereadily apparent from the following detailed description, simply byillustrating a number of particular embodiments and implementations,including the best mode contemplated for carrying out the invention. Theinvention is also capable of other and different embodiments, and itsseveral details can be modified in various obvious respects, all withoutdeparting from the spirit and scope of the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which

FIGS. 1A-1C are diagrams that illustrates a microfluidic flow cytometer,according to an embodiment;

FIG. 2A and FIG. 2B are flow profile diagrams that illustrate effect ofchannel width on shear, according to an embodiment;

FIG. 3A and FIG. 3B are diagrams that illustrate an apparatus fordynamic electromagnetic focusing, according to an embodiment;

FIG. 4A and FIG. 4B are diagrams that illustrate electromagneticmeasurement apparatus and data, according to an embodiment;

FIG. 5A is a block diagram that illustrates in plan view an examplemicrofluidic electromagnetic measurement apparatus, according to anembodiment;

FIG. 5B is a block diagram that illustrates in partial cross-sectionview an example microfluidic impedance measurement apparatus, accordingto an embodiment;

FIG. 5C through FIG. 5G are diagrams that illustrate an apparatus fordynamic electromagnetic focusing and multiple electromagneticmeasurements, according to another embodiment;

FIG. 6A is a diagram that illustrates channel engineering to stabilizetwo-phase flows, according to an embodiment;

FIG. 6B and FIG. 6C are block diagrams that illustrates example channelcross sections for a microfluidic electromagnetic measurement apparatus,according to various embodiments;

FIG. 6D and FIG. 6E are micrographs that illustrate core flow in thepresence of example rails and electrodes, respectively, according to anembodiment;

FIGS. 7A through 7C are diagrams that illustrate core modulationcontrolled by a flow rates or pressures or both; according to variousembodiments;

FIG. 7D is a graph that illustrates example core width variability basedon alternative flow driving mechanisms, according to variousembodiments;

FIG. 8 is a diagram that illustrates optical confinement of light incore, according to an embodiment;

FIGS. 9A and 9B are micrographs that illustrate resting platelet and anactivated platelet, respectively;

FIG. 10 and is a graph of simulated data of the impedance spectrum of amixed population of activated and resting platelets, according to anembodiment;

FIGS. 11A through 11F are graphs of simulated histogram data of theimpedance at various frequencies for platelet populations activated tovarious degree with adenosine diphosphate (ADP), according to anembodiment;

FIG. 12 is a diagram that illustrates electromagnetic measurementapparatus for multiple frequencies, according to an embodiment;

FIG. 13 is a block diagram that illustrates a computer system upon whichan embodiment of the invention may be implemented;

FIG. 14 illustrates a chip set upon which an embodiment of the inventionmay be implemented;

FIG. 15A and FIG. 15B are flow charts that illustrate an example methodfor determining condition of a subject based on a microfluidic device,according to various embodiments;

FIGS. 16A and 16B are graphs that illustrate example detection ofparticles in a core flow, according to an embodiment;

FIGS. 17A and 17B are graphs that illustrate example frequency responseof resting and activated platelets, according to an embodiment;

FIG. 18 is a graph that illustrates example classification of restingand activated platelets, according to an embodiment;

FIG. 19 is graph that illustrates example agreement of degree ofplatelet activation with a standard, according to an embodiment; and

FIG. 20A and FIG. 20B are graphs that illustrate example detection ofplatelets and red blood cells in a blood sample, according to anembodiment.

DETAILED DESCRIPTION

A method and apparatus are described for enhanced microfluidicelectrical measurements. In the following description, for the purposesof explanation, numerous specific details are set forth in order toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form inorder to avoid unnecessarily obscuring the present invention.

Some embodiments of the invention are described below in the context ofimpedance measurements of platelets in a high dielectric fluid inmicrofluidic channels of a particular range of sizes. However, theinvention is not limited to this context. In other embodiments the sameor different electrical properties of the same or different particlesare measured in microfluidic channels of the same or different sizes.The proposed methods bring a substantial gain in sensitivity overtraditional microfluidic approaches, while minimizing the drawbacks ofsmall channel size (high shear stress on cells, channel clogging). Theseimprovements pave the way for the characterization of very smallparticles (<1 micron, 1 micron=10⁻⁶ meters), and particles that aresensitive to shear stress (e.g., platelets).

A microfluidic channel, also called a microchannel herein, is a channelfor fluid flow with a width and height each less than 1000 microns (alsocalled micrometers, μm). Fluid flows in microfluidic channels areusually laminar rather than turbulent and have comparatively lowReynolds number values (<1), both situations resulting from themicrochannel dimensions, the flow rates, and the fluid properties.

A used herein, a particle is any single object in motion within a fluidflow, which is small enough to fit within a microfluidic channel andlarge enough to affect an electromagnetic measurement of a fluidproperty within the channel. Particles include cells (includingbacteria) and portions thereof, including platelets, as well as dust,pollen and other organic and inorganic materials.

1. OVERVIEW

In this section, an overview of the methods and apparati are given. Moredetailed embodiments are described in other sections. Electricalimpedance, represented by the symbol Z, is a well known measure ofopposition to alternating current (AC) and combines the effects ofelectrical resistance for direct current (DC) and phase shifts for (AC).Some detailed embodiments are described for electric impedancemeasurements of platelets, including a description of a prototypeapparatus for measuring the electric impedance of platelets. Eachreference cited herein is hereby incorporated by reference as if fullyset forth herein, except so far as the terminology is inconsistent withthe terminology used herein.

FIGS. 1A-1C are diagrams that illustrate a microfluidic flow cytometer,according to an embodiment. FIG. 1A shows a perspective cutaway view ofa microfluidic channel 110 (also called microchannel herein) formedbetween two side walls 102, a top wall 104, and a bottom wall 106. Forsome electromagnetic measurements, such as impedance, one or moreelectrodes 120 are included on one or both opposite walls, e.g., a topwall and bottom wall in FIG. 1A. FIG. 1B shows a cross section acrossthe width of the channel. FIG. 1C shows a cross section along a portionof the length of the channel. Impedance measured between the pair ofelectrodes is related to the total impedance of the particle 132 and thefluid media 130 between the electrodes 120. An impedance model 140 for acell is depicted, with different contributions from cell wall and cellbody represented by different boxes that represent different parametersof the model. Values for one or more parameters are estimated based onthe impedance measurements at one or more alternating current (AC)frequencies.

To first order, the magnitude of an electromagnetic signal associatedwith the passage of a particle (including electrical impedance andoptical interaction) is related to the size of the particle relative tothe size of the channel in which the particle is flowing. If theparticle blocks most of the channel when it is between the sensors (suchas electrodes), the signal due to the passing particle is comparativelylarger, and the property can be measured with higher resolution. If theparticle is small compared to the channel, then the signal iscomparatively weaker, reducing the resolution. This means that, forsmall particles, the channel is preferably small in order to produce adetectable change in signal. A further disadvantage of large channels isthat such large channels can increase the probability of having morethan one particle (co-occurrence) in the detection region of the sensor,e.g., between the detection electrodes, which could compromise thedetection.

Unfortunately, small microfluidic channels (widths or heights <20 μm)imply large shear forces (e.g., compared to shear forces in the venousportions of an uncompromised circulatory system) across particles due tothe typical parabolic flow profile in pressure-driven flows. FIG. 2A andFIG. 2B are flow profile diagrams that illustrate the effect of channelwidth on shear, according to an embodiment. Flow direction 131 is asdepicted. Flow profile shows velocity profiles 151, 152, respectively,as a function of distance from a boundary, such as a channel wall 102.Velocity is given by the length of an arrow touching the paraboliccurve. When width is small as in FIG. 2A, shear, a measure in thiscontext of gradients perpendicular to a wall in velocity, is great. Thesame rate of flow through a wider channel, as in FIG. 2B, produces muchsmaller shears.

Small channels also are very prone to occlusion when dealing withsamples having significant particle content (e.g., blood). Inparticular, for specific applications where shear rates have to beminimized (e.g., blood platelets, which are activated by shear),optimization of the channel size for signal strength (small channel)conflicts with the desire for a large channel to minimize shear values.

To circumvent these limitations, several embodiments usedielectric-based hydrodynamic focusing in order to allow the use of alarge physical channel (hence reduced shear rates), with thesimultaneous advantage of a small electrically conductive channel thatcarries the particles. While conductivity is the primary parameterdefining the field and current path through a structure at directcurrent (DC) and low alternating current (AC) frequencies, thedielectric constant and related permittivity are the dominant factors athigh AC frequencies. The exact frequency at which dielectric propertiesstart to dominate over conductivity properties is dependent on a numberof factors (e.g., actual electrical parameters of the materials andstructure, geometry, parasitic capacitance/resistance, etc.), but onlyan appropriate control of the dielectric/permittivity characteristics ofthe channel would allow the contrast between the physical channel andthe electrically conductive channel to be maintained over an extendedfrequency range.

The dielectric constant is a dimensionless ratio of the permittivity ofa substance to the permittivity of free space. It is an expression ofthe extent to which a material concentrates electric flux. Permittivityis determined by the ability of a material to polarize in response tothe electric field, and thereby reduce the total electric field insidethe material. Thus, permittivity relates to a material's ability totransmit (or “permit”) an electric field (in particular an alternatingelectric field); and is dependent on a number of factors, including ACfrequency. Thus, the electric field between two plates passespreferentially through a material with high permittivity (largedielectric constant) and around a material with low permittivity (smalldielectric constant). For example, for impedance measurements, theparticles are included in a core fluid with relative large dielectricconstant and kept apart from the walls of the channel by a differentfluid with a relatively small dielectric constant. The different fluidforms a sheath around the core fluid and is called hereinafter thesheath fluid. Therefore the electric field between the electrodes inopposite walls of the channel passes predominately through the corefluid rather than the sheath fluids. In other embodiments, the sheathfluid has different values of other electromagnetic properties, such asdifferent values for an index of optical refraction. As used herein thecore fluid refers to the material flowing in a core flow, and the core,or core flow, refers to the spatial distribution of the core fluid,having such properties as position, width, height, flow rate (mass perunit time) and velocity.

This concept is illustrated in FIG. 3A and FIG. 3B, which are diagramsthat illustrate an apparatus for dynamic electromagnetic focusing,according to an embodiment. FIG. 3A depicts a plan view of a centralchannel 310 and two side channels 312. The target of the electromagneticmeasurements is one or more particles in a core fluid introduced intothe central channel at the sample inlet 320. The sheath fluid with thedifferent value of an electromagnetic property, e.g., a much lowerdielectric constant, is introduced at the sheath flow inlets. The fluidsmerge at a junction 314; and the core fluid is confined to a core flowin a central portion of the channel downstream of the junction, wellaway from the channel walls, and, thus, in a portion of the flow profilewhere shear is near zero. The sensor for the electromagneticmeasurements, such as a set of electrodes 120 on top and bottom walls,is also downstream of the junction 314. The flows leave the device atoutlet 316.

The effect of a low dielectric sheath fluid in the cross section at thesensor is depicted in FIG. 3B, where the sheath fluid is light gray, andthe core fluid is darker gray, and a particle 132 of size comparable tothe width of the core fluid is shown as a white oval. Electric fieldlines 330 between electrodes 120 are depicted as downward pointingarrows, with flux decreasing as spacing between arrows increases. Theelectric field flux is predominately through the core fluid and theparticle. The cross-section emphasizes the localization of the sensingcurrent in the core (as symbolized by arrow density) due to thedielectric constant (and thus conductivity) contrast between the coreand the sheath.

For core and sheath flow of similar viscosities, the flow profile acrossthe channel will be similar to the classic parabolic flow of a largechannel. Particles or cells traveling in the core are thus subjected tolittle shear. However, since the core is more conductive, the sensingcurrent for the impedance measurement is confined to the core, mimickingan electrically conductive channel of the width of the core.

In the illustrated embodiment, it is very advantageous to use a sheathfluid with a very low dielectric constant (e.g., fluorocarbon solvent ormineral oil), which provides electromagnetic focusing over a much largerrange of AC frequencies than a water-based sheath most often describedin the literature. For instance, a perfluorocarbon sheath (3MFluorinert™ FC Series, ∈_(r)=1.9, ρ>10¹⁵ Ω·cm, where ∈_(r) isdimensionless relative permittivity and ρ is linear DC resistance in ohmcentimeters, Ω·cm, and 1 cm=10⁻² meters) in a 50×50 μm channel (50microns wide and 50 microns high) with a 10 μm wide core and 10 μm longelectrodes, extends the usable AC frequency range for impedance from 70megahertz (MHz, 1 MHz=10⁶ Hertz, 1 Hz=1 cycle per second) for adeionized water sheath (∈_(r)=80, ρ˜10¹⁸ Ω·cm) to more than 360 MHz forthe perfluorocarbon sheath.

In addition, the use of a low dielectric constant liquid immiscible inwater (e.g., fluorocarbon solvents or oils such as mineral oil) for thesheath fluid provides an abrupt boundary at the sheath/core interface,as opposed to aqueous-based sheaths, which are subject to diffusion and‘blurring’ of the interface. Such immiscible liquids comprisepredominately non-polar molecules and are said to be hydrophobic. Sheathfluids of different molecular polarity from the core fluid (calledtwo-phase flows herein) thus offer the advantage of a sharp boundary andfurther offer the advantage of producing small fluid core widths (<10μm) most useful to measure small particles (<1-5 μm) such as platelets.

A disadvantage of two-phase flows is difficulty found in maintaining astable flow boundary. An unstable flow boundary leads to variability inthe width of the core and variability in the electromagnetic measurementthat is not related to the passage of particles. In various embodimentsdescribed in more detail below, adjustments to the apparatus or methodsor both are made to decrease the disadvantageous effects of unstablecore width in two-phase flows.

While this hydrodynamic focusing is only along one dimension in theillustrated embodiment, it is enough to reduce the cross-section of thesensing volume to a size comparable to the size of the particle to besensed. The vertical dimension is not critical with this approach, andcan be kept large, thus reducing shear rates in the vertical direction.

In some embodiments the particles or core fluid or both are activelycentered in the channel (in the region of lowest shear) using well-knownmethods such as dielectrophoresis. Such active centering further reducesthe variability of the measurements and the shear rates to which theparticles are subjected.

Another benefit of electromagnetic property focusing is the possibilityof achieving the narrow fluid cores (required for high resolution) inwide channels produced by low-cost techniques, such as laser-cutchannels in pressure-sensitive adhesive. These less stringentrequirements with respect to channel geometry mean simpler, cheaperfabrication methods can provide measurement resolution comparable toconventional but expensive, lithographically-defined microchannels.

Any method can be used to make the electromagnetic measurements. Forexample, in a simple embodiment the electrical impedance is measured asdepicted in FIGS. 4A and 4B. FIG. 4A and FIG. 4B are diagrams thatillustrate electromagnetic measurement apparatus and data, according toan embodiment. FIG. 4A depicts an example electrical measurementapparatus using two pairs of electrodes 402 a, 402 b. In thisembodiment, particle spacing in the core fluid is controlled so that thetypical distance between particles is greater than the distance betweenthe two pairs of electrodes. Flow direction is represented by flow 404.Thus when the particle is in the detection region for the first pair ofelectrodes, no particle is in the detection region of the second pair.The impedance difference between the two pairs is dependent on theimpedance of the particle Rp 412, and independent of the impedance ofthe channel, Rc 414.

The apparatus includes, besides the electrodes 402 a, 402 b and thechannel carrying the fluids, an excitation module 420, a differentialamplifier 422, a synchronous demodulator 424 and a computer 426. The ACfrequency of the measurement is determined by the excitation module.Multiple frequencies are usually superimposed in the excitation signalfrom the excitation module. The current measured through the electrodeson opposite walls is inversely proportional to the impedance. Thecurrent difference is amplified by the differential amplifier and ACphase determined in the synchronous demodulator. The measurements aresent to a computer, such as general purpose computer depicted in FIG. 13or chip set depicted in FIG. 14, where program instructions are used toderive the impedance of the cell based on the electrical current phaseand amplitude differences at the two pairs of electrodes. By using twopairs of electrodes, a differential measurement is performed, reducingthe sensitivity to common-mode noise. FIG. 4B is graph 450, with time onhorizontal axis 452 and impedance change on vertical axis 454. Theimpedance data are graphed in FIG. 4B as trace 460 showing one impedancechange as the particle first passes the first pair of electrodes andthen the opposite effect when the particle passes the second pair ofelectrodes. In some embodiments, multiple AC frequencies are summed atthe excitation module to measure impedance at multiple frequenciessimultaneously on the same electrode pairs.

In some embodiments, enabled by micro-fabrication techniques, impedancemeasurements at multiple AC frequencies rely on the use of multiple setsof electrodes along the channel, each set including one or more pairs ofelectrodes and measuring the same particle as it flows through the set.Such embodiments provide independent measurements of the impedance ateach set, allowing averaging or other statistical approaches to be usedfor increasing the signal-to-noise ratio (SNR). For instance, if theparticle is measured by four sets of independent electrodes, then theaveraging of these four measurements increases the SNR by two. In someembodiments, using multiple measurements along the channel also averagesout the impedance variation due to possible rotation of a non-sphericalparticle within the core. Using other sets of electrodes as referencealso allows adaptive signal processing to be used, such as for noiseremoval, in some embodiments. Furthermore, in some embodiments, thevarious sets of electrodes are used to probe different frequencies. Suchsequential measurements of the same particle along the channel are notobviously available in typical aqueous flows, due to the rapid diffusionand degradation of the core profile. Such sequential measurements aremuch more feasible with two-phase flows, which conserve a well-definedcore over long distance even at low flow rates.

The microfluidic flow cytometer can be coupled to other fluidicfunctions, such as cell sorting or inline chemical activation/treatmentof cells prior to or after these electromagnetic measurements.

As described above, with more details to follow below, dielectricfocusing provides many advantages. When combined with impedancespectroscopy, these techniques provide increased signal-to-noise ratioover an extended frequency range (>100 MHz). Low-shear-rate flowcytometry apparatus can still be used to obtain high impedanceresolution. Two-phase (polar/non-polar) flow further improves dielectricfocusing by limiting diffusion and allowing the use ofultra-low-dielectric-coefficient liquids (e.g., fluorocarbon solventsand mineral oil). High-resolution impedance spectroscopy is thereforepossible in even wider, simpler-to-fabricate channels.

In an example embodiment, impedance spectroscopy of platelets isdemonstrated in a microfluidic flow cytometer for the evaluation ofplatelet activation levels without disturbing platelet activation stateby high shear flows. In some embodiments, the use of air sheath orhydrophilic tracks in an otherwise-hydrophobic channel is shown toconfine the aqueous core, thus providing flow with minimal shear rate atthe air-water interface, yet with increased electrical confinement.Precise and stable core width is achieved in some embodiments throughclosed-loop control of sheath and core flows based on continuousimpedance measurement. In some embodiments inner wall surface features,such as rails, are effectively employed to stabilize two-phase flows.Such stabilization can be applied to both aqueous flows and two-phaseflows, and could also be used in some embodiments as a calibration steprather than for real-time control. Sequential measurements of the sameparticle along the channel using multiple independent sets of electrodesallows separation of AC frequencies or application of signal processingtechnique to increase information content (e.g., higher SNR throughaveraging, adaptive filtering). Particles with known electromagneticproperties are also used in some embodiments to track the actualvariability of the core flow.

Use of low-dielectric sheath fluid (with low refractive index) is alsouseful to guide light for optical interrogation of particles. Thelow-refractive-index sheath with the higher-refractive-index coreprovides in effect an optical waveguide to focus the light beam onto theparticles in the core. The shape of the core can be assessed bymeasurement of total internal reflection. In various embodiments, suchoptical measurements with index of refraction focusing are correlatedwith simultaneously-acquired electrical (e.g., impedance) measurements,with or without dielectric focusing. In various embodiments, opticalmeasurements that do not rely on total internal reflection are used aswell as or instead of optical measurements that do rely on totalinternal reflection, with or without electrical measurements, or with orwithout dielectric focusing.

2. EXAMPLE APPARATUS EMBODIMENTS

FIG. 5A is a block diagram that illustrates an example microfluidicelectromagnetic measurement apparatus 500, according to an embodiment.The apparatus 500 includes microchannels 502 through which a sampleinput port 504 is connected to a central microchannel that joins twosheath microchannels each fed by a sheath fluid input port 506 a, 506 b.The microchannels from each input port meet at a junction which connectsto an exit port 508 through a portion of the central microchannel. Insome embodiments, additional sample or sheath microchannels areincluded.

The sample to be analyzed is introduced at the sample input port 504,and flows along the central microchannel 502 to the junction. One ormore sheath fluids are introduced at the sheath fluid input ports 506 a,506 b and flows along the side microchannels 502 to the junction.Downstream of the junction the sample fluid forms a core flow betweenthe sheath fluid flows and all fluids exit at the exit port 508.

The apparatus further comprises one or more pressure actuators (such aspressure controllers or pressure regulators) to apply fluid pressure atone or more input or exit ports. For example, in the illustratedembodiment, apparatus 500 includes a sample pressure actuator 514 thatacts at sample input port 504, and two sheath fluid pressure actuators516 a, 516 b that act at sheath input ports 506 a, 506 b, respectively.Any method may be used to apply pressure at the various input ports, forexample a servo motor controlling a syringe can be used, compressed airexerting a pneumatic pressure on the sample and sheath fluid in areservoir, hydrostatic pressure due to a fluid reservoir at fixedelevation above the main apparatus, or a diaphragm controlled byhydraulics. In an experimental embodiment, these pressures are appliedto the sample and sheath inlets, with the outlet remaining atatmospheric pressure. In various embodiments the same or differentpressure is applied at each input port. In some embodiments, some or allof the actuators are on the chip with the microchannels. In otherembodiments, the actuators are external to the chip that includes themicrochannels. In some embodiments, one or more pressure actuators 514,516 a, 516 b are omitted.

The apparatus 500 includes a processor/controller 520, such as amicroprocessor or general purpose computer with zero or more applicationspecific integrated circuits (ASICs) programmed to control variouscomponents of the apparatus. In some embodiments, theprocessor/controller 520 controls the pressure actuators 514, 516 a and516 b. In some embodiments some or all of the processor/controller is onthe chip with the microchannels. In other embodiments, the entireprocessor/controller is external to the chip that includes themicrochannels. In an experimental embodiment, pressures was eithergenerated by stand-alone syringe pumps (Cole-Parmer), manually set to afixed flow rate, height (hydrostatic pressure from manually raisedsample and sheath fluid reservoir), or pneumatically (compressed air),in which case the pressure was regulated by a combination of pressuresensor and proportional pneumatic valve in a closed-loop system(microcontroller).

The apparatus 500 includes one or more sample conditioning components530 to condition the sample fluid before it becomes the core fluiddownstream of the junction. For example, reservoirs of one or morereagents are included in conditioning components 530, such as reagentsthat stimulate activation of platelets. In some embodiments, chemical ormechanical filters, mixers or buffers are included in the sampleconditioning components 530, such as mechanical filters for separatingplatelets from red blood cells. In some embodiments, the sampleconditioning components include a source of known particles 540 that areused to determine or correct for variability in core width, or both, asdescribed in more detail in a later section. In an experimentalembodiment, sample preparation (dilution, reagent mixing, addition ofcalibration particles) was performed manually in separate containers,then connected to the apparatus 500 for analysis.] In some embodimentssome or all of the sample conditioning components 530 are on the chipwith the microchannels. In other embodiments, all the sampleconditioning components 530 are external to the chip that includes themicrochannels. In some embodiments, the sample conditioning components530 are omitted; and, any known particles 540 are included in the samplefluid at the input port 504. In some embodiments, the sampleconditioning components 530 are controlled, in whole or in part, by theprocessor/controller 520.

The apparatus 500 includes one or more centering components to centerparticles in the sample fluid in the core flow. For example, in theillustrated embodiment, the apparatus 500 includes dielectrophoresiscomponents 550, e.g., to drive particles toward the center of the coreflow. In some embodiments the centering components, such asdielectrophoresis components 550, are omitted. In some embodiments, thecentering components, such as dielectrophoresis components 550 arecontrolled, in whole or in part, by the processor/controller 520.

The apparatus includes one or more core fluid sensors 560, such aselectrodes, as described above, or optical sensors. The core fluidsensors 560 produce or detect, or both, electromagnetic signals that,owing to the preferential passage of the electromagnetic signals throughthe core (e.g., by dielectric focusing), are more sensitive toperturbations that occur in the core fluid, such as the passage of aparticle. In some embodiments, the core fluid sensors 560 are monitoredor controlled, in whole or in part, by the processor/controller 520.

FIG. 5B is a block diagram that illustrates in partial cross-sectionview an example microfluidic impedance measurement apparatus, accordingto an embodiment. In this embodiment, a custom impedance spectroscopysystem is composed of a lock-in detector (SR830 DSP, Stanford ResearchSystems, Sunnyvale, Calif.) 564, a custom differential current-voltageconverter 566, and a computer-controlled acquisition system 520. Thissystem gave satisfactory results; however, it was limited in frequencyto 100 kHz. For measurements at higher frequencies, a multi-frequencyimpedance spectroscopy system from Zurich Instruments was used, andgreatly improved measurement capabilities in other embodiments. Thissystem was capable of simultaneously measuring six frequencies up to 50MHz (10 MHz effectively once parasitic capacitance of various chipcomponents was taken into account). All platelet characterizations,described in a later section, were carried out using this system.Typical excitation voltages were kept under 1 Volt (V) per excitationfrequency to avoid heating of the sample. Sampling speed was 7.8 kHz inorder to fully capture the transient impedance change.

2.1 Multiple Sets of Detection Electrodes

FIG. 5C through FIG. 5G are diagrams that illustrate an apparatus 570for dynamic electromagnetic focusing and multiple electromagneticmeasurements, according to another embodiment. The example microfluidicimpedance cytometer uses a fluidic layer fabricated from 50-μm-thick,double-sided adhesive film (Adhesives Research, Glen Rock, Pa.) using aCO₂ laser engraving and cutting system (Universal Laser Systems,Scottsdale, Ariz.). The device 570 includes sample inlet 571 and twosheath inlets 572 a, 572 b and focusing electrodes 573 and outlet 578.Three sets of electrodes 574 (for single or repeated measurements), eachcomprising two pairs of 20-μm-wide platinum electrodes, separated by 100μm, patterned on Borofloat® glass using standard photolithography andsputtering, were used for differential measurements. The fluidic layerwas sandwiched between two glass chips 582 (see FIG. 5D) and bondedusing a hot plate at 70° C. and a hand roller. Fluidic connections werefabricated from polydimethylsiloxane (PDMS) and bonded to the top glasschip with the aid of air plasma that was used to chemically activate thesurface to improve adhesion.

FIG. 5C shows a plan view of the channel layer with two sheath inletchannels 572 a, 572 b flanking a central sample inlet channel 571 thatmeet at a common junction beyond which is a common channel ending in anoutlet 578. The inlet channels are about 10-millimeter long (mm, 1mm=10⁻³ meters), as is the common channel. In the common channeldownstream of the junction are focusing electrodes 573 that cause theparticles to flow near or at the center of the common channel usingdielectrophoresis. Farther downstream are three sets of detectionelectrodes 574, each set comprising two pair of electrodes forcomparative measurements of each particle. The common channel narrowsappreciably in the vicinity of the detection electrodes from 1000 μm to350 μm in order to facilitate the focusing of narrow cores.

FIG. 5D shows a cross sectional view in a plane perpendicular the longdimension of the channel. The channel is 50 μm high, 350 μm wide with ahigh conductive and high dielectric constant core fluid 583 occupying acore flow only about 20 μm wide at the center of the channel and flankedby dielectric sheath fluids 585. The top and bottom walls are glasswafers 582 upon which one or more of the electrodes are supported.

FIG. 5E depicts three dimensional (3D) computed aided design (CAD)drawings of the microfluidic chip exploded by layers 590 a, 590 b, 590c. FIG. 5F is a photograph of bonded chip 592. FIG. 5G is a photographof a complete example microfluidic cartridge 594 with ports 596 forfluidic connections and electrical contacts.

2.2 Channels for Two-Phase Flows

In some embodiments, as described above, two-phase flows are used. Forexample, in some embodiments an aqueous core fluid (polar watermolecules) is used with sheath fluids comprising non-polar molecules.FIG. 6D and FIG. 6E are micrographs that illustrate core flow in thepresence of example rails and electrodes, respectively, according to anembodiment. FIG. 6E shows an example of such a flow, with a plan view ofelectrodes 678 and a core fluid 680 flowing between immiscible sheathfluids 684. A 100 micron scale bar 694 is depicted. However, achievingstable, reproducible two-phase flows in microchannels has beennotoriously difficult, and most of the solutions found in literaturerequire the use of high-viscosity oils as sheath fluid, generating largeshear at the interface core/sheath. In order to allow the use of lessviscous fluid (such as fluorocarbon solvents with very low dielectricconstants), a method based on work by Zhao, Moore and Beebe (AnalyticalChemistry, Vol. 74, No. 16, Aug. 15, 2002) using surface energypatterning is used in some embodiments, as depicted in FIG. 6A.

FIG. 6A is a diagram that illustrates channel engineering to stabilizetwo-phase flows, according to an embodiment. In channel formed by sidewalls 602 a, 602 b, top wall 604 and bottom wall 606, an aqueous corefluid 610 is guided by high-surface-energy (hydrophilic, γ₂) tracks 622in a low-surface-energy (hydrophobic γ₁) coated 624 channel. In someembodiments, these tracks are patterned by prior hydrodynamic focusingwith fluids containing surface modification chemicals, resulting in aself-aligned process. In some embodiments, the sheath fluid is simplyair (or another gas), simplifying the fluidics and reducing shear ratesat the core/sheath interface. It is believed that the application ofsuch a gas as a sheath fluid to provide reduced shear rates has not beensuggested yet.

For example, in some embodiments, the channel surface modification areself-aligned using hydrodynamically-focused laminar flows containinghydrophobic (non-polar) molecules in a sheath flow and hydrophilicmolecules in a core flow. These molecules are selected so that theyattach to the surface as they flow through the channel. Once the surfacehas been modified, the channel is flushed and the fluids and particlesof interest for measurement are introduced into the device. Thehydrophilic ‘lane’ then stabilizes and guides the aqueous core in itsdesired path along the center of the channel. The guiding effectprovided by the patterned surface energies can also be augmented bytopographical cues (slight height difference between the high and lowsurface energy coatings). Including such surface patterning provides ageometrically stable and spatially reproducible core.

Experiments were performed using this surface chemistry approach. Insome embodiments, standard silane chemistries (combinations offluorosilanes and aminosilanes), as well as Bovine Serum Albumin (BSA)or Pluronic F68 (an amphiphilic copolymer from BASF, Ludwigshafen,Germany) adsorption were used. In some embodiments, a micro-stampingmethod was used as a way to define the hydrophobic/hydrophilic track.

In some embodiments, topographical cues were used, alone or incombination with surface chemistry. FIG. 6B and FIG. 6C are blockdiagrams that illustrates example channel cross sections for amicrofluidic electromagnetic measurement apparatus, according to variousembodiment. FIG. 6B shows a single rail 670 surface topography on bothtop and bottom walls. In this embodiment, a boundary between a corefluid 680 and each immiscible sheath fluid 684 is expected to follow anedge of the single rail 670. FIG. 6C shows multiple rail 672 surfacetopography. In this embodiment, a boundary between a core fluid 680 andeach immiscible sheath fluid 684 is expected to follow an edge of onerail of the multiple rails 672. For example, tracks and ridges weredefined in photoresist or SU-8 along the channel to physically guide thecore. These methods were successful in bare channels. Thus rails wereobserved to provide an advantage in stabilizing two-phase flows. In someembodiments, the rails are added to only one of the top and bottomwalls. FIG. 6D shows a plan view of a example rail 670 and a core fluid680 flowing between immiscible sheath fluids 684, according to anembodiment. A 50 micron scale bar 692 is depicted.

However, the addition of electrodes on the surface noticeably disruptedthe flow. Ultimately, a combination of topographical and surfacemodification is anticipated to achieve a reliable, stable two-phase flowat those dimensions and flow rates.

2.3 Dynamic Modulation of Core Width

In some embodiments, the width of the core is modulated to some extentdynamically by varying the differential pressure between the core andthe sheath. In some embodiments, this principle is used to provide avariable width of the flow of the core fluid (called the core flowhereinafter). Such embodiments allow rapid reconfiguration of core flowwidth for optimally sensing particles of different sizes, all with thebenefits of low shear and high resolution. In some embodiments, theimpedance of the core, as measured by the detection electrodes, is usedto estimate its width. For example, the impedance of the core absent aparticle, or the average minimum impedance over a time interval duringwhich one or more particles have passed, is used to determine width. Insome embodiments, a width calibration curve is generated throughexperiment. In other embodiments other properties are measured, such asthe optical measurements of core properties described in the nextsection.

In some of these embodiments, the measurements, such as the impedance oroptical measurements are used to control the flows (such as bycontrolling the pressure) in a feedback loop to set precisely the coreto a pre-determined width. For example, by maintaining or adjusting thepressure to stabilize an optical or impedance measurement at apredetermined value associated with the desired width. In variousembodiments, such control is implemented in a separate calibration step,or is accomplished in real time (e.g., determining an average minimumvalue for a time interval).

In some embodiments, the width of the core flow is calibrated based onthe measured impedance of the known particles 540 depicted in FIG. 5A,such as mono-disperse polystyrene beads. Such beads, of calibrateddiameters, generate a control signal that can be used to estimate coreproperties, or normalize the signal (as described in more detail below).This normalization accounts for varying flow rate, core flow width dueto boundary instability between oil sheath and core fluid, and corefluid conductivity differences of different samples.

FIGS. 7A through 7C are diagrams that illustrate core modulationcontrolled by flow rates or pressures or both; according to variousembodiments. FIG. 7A shows a wide core flow 710 caused by a positivecore pressure 712(P2) greater than sheath flow pressure 714 (P1), and anassociated impedance value 716 Z1 absent a particle (e.g., at a minimummeasured impedance). FIG. 7B shows a moderate width core flow 720 causedby a small relative pressure of core pressure 722 (P2) about equal tosheath flow pressure 724 (P1), and an associated impedance value Z2 726absent a particle. FIG. 7C shows a narrow core flow 730 caused by anegative relative pressure of core pressure 732 (P2) less than sheathflow pressure 734 (P1), and an associated impedance value 736 Z3 absenta particle. By decreasing the sheath flow pressure (P1) when theimpedance absent a particle is between Z2 and Z3, and increasing thesheath flow pressure (P1) when the impedance absent a particle isbetween Z1 and Z2, the core width can be stabilized to that shown inFIG. 7B.

Because width is the narrowest dimension of the core flow, theseembodiments amount to controlling a narrowest spatial dimension of thecore fluid by controlling relative pressure of a source of the corefluid compared to pressure of a source of one or more of the sheathfluids. To stabilize the core width, some of these embodiments involvecontrolling the relative pressure to stabilize a measurement of aproperty of the core fluid, such as its impedance, its impedance absenta particle (e.g., average minimum impedance during a time interval) oroptical measurements of width, alone or in some combination.

Experiments were performed that showed pressure-induced flow provided anadvantage over pulsatile syringe-pump-induced flow used in someembodiments. As expected, flows generated with syringe pumps exhibitedpulsatile behavior due to the stepping action of the motors that drivethese pumps. Because of the differential nature of the measurement, theimpact of these pulsations was strongly reduced. However, thesepulsations were still contributing to some residual instabilities in theflow, and likely to some of the variability in the final data. Toimprove on this, pressure-controlled flows were implemented for thesheaths (the most sensitive to pulsation) in some embodiments. In oneembodiment, hydrostatic pressure was generated by raising the fluidreservoir (gravity-flow) between 1 and 20 cm above the chip level(usually by filling a pipette to the desired height). In a secondembodiment, a closed-loop system was used to provide constant fluidpressure. This system used a proportional pneumatic valve to pressurizethe fluid reservoir with air. A pressure sensor in line with the fluidconnection to the chip was sensing the fluid pressure, and feeding it toa closed-loop control system regulating the proportional valve. Thissystem effectively regulated sheath inlet pressure with a fixed,selectable pressure.

This approach greatly reduced signal variation, and improved overallstability of the core. FIG. 7D is a graph 770 that illustrates examplecore width variability based on alternative flow driving mechanisms,according to various embodiments. The horizontal axis 772 is time inseconds (s); the vertical axis 774 is voltage proportional to deviationsin width of the core flow. Traces are shown for syringe-driven flowsthat exhibit a large variability 782 in widths. In contrast, traces areshown for pressure-driven flows that have much lower variability 784 ofwidths. Based on these data, any non-pulsatile, constant-pressurefluidic system is advantageous for future implementation. In addition tosimplifying sample handling, some implementations of such an approachhave also the merit of being more easily integrated into a point of care(POC) system design.

2.4 Optical Measurements of Core Flow

The concept of electromagnetic property focusing can be used not only tofocus electrical currents, but also, or alternatively, light beams. Byusing a core with a higher refractive index than the sheath (a conditionreadily obtained with an aqueous core and a fluorocarbon sheath), onecan achieve the conditions for total internal reflection of light insidethe core, depending on the angle of incidence of the light. Thisoptimally focuses the light within the core, where it can be used tointerrogate passing particles. Any optical measurement can be used invarious embodiments, such as fluorescence, scattering, absorption,diffraction or other optical interactions, with or without totalinternal reflection, with or without impedance measurements, and with orwithout dielectric focusing.

FIG. 8 is a diagram that illustrates optical confinement of light in thecore, according to an embodiment. The index of refraction in the sheathfluid 824 is n1, in the core 810 is n2, and in the bottom wall 806 isn3. The core fluid has a higher refractive index than the sheath fluid(n₂>n₁). With both n2 and n3 greater than n1 of the sheath, light isreflected away from the sheath fluid. With n2 and n3 similar in value,light easily enters the core which acts as wave guide, emitting most ofthe light into the top wall 804 with an index of refraction also closeto that of the core fluid. Side walls 802 a, 802 b can have any index ofrefraction. Since refractive index is directly related to relativepermittivity, the conditions for good electrical focusing (lowdielectric sheath) are ideally suited for such optical confinement,allowing simultaneous optical and electrical interrogation.

Optical interrogation can then be coupled to electrical interrogation toprovide enhanced information on the measured particles. In a similarapplication to that shown in FIGS. 7A through 7C, optical interrogationcan also be used to evaluate the width of the core, since internalreflection will be modified by the angle of the core/sheath interfaces.In some embodiments, optical focusing in the core is used even withoutdielectric focusing in the core.

3. EXAMPLE METHOD

FIG. 15A and FIG. 15B are flow charts that illustrate an example methodfor determining condition of a subject based on a microfluidic device,according to various embodiments. Although steps are depicted in FIG.15A and FIG. 15B as integral steps in a particular order for purposes ofillustration, in other embodiments, one or more steps, or portionsthereof, are performed in a different order, or overlapping in time, inseries or in parallel, or are omitted, or one or more additional stepsare added, or the method is changed in some combination of ways.

In step 1501, a sample is collected from a subject. For example, a bloodsample is collected from a patient. In a calibration embodiment, asample with known values of one or more properties, such as knownpercentage of activated platelets, is collected during step 1501.

In step 1503, the sample is prepared prior to introduction to theapparatus. For example, the sample is diluted or filtered or prepped inany other manner known in the art. In one embodiment, the blood sampleis centrifuged to separate the platelet rich plasma (PRP). The plateletsare then re-suspended in Tyrode's buffer with added chemicals (such asbovine serum albumin, acid citrate dextrose, apyrase) to limitaggregation and activation. In some calibration embodiment, the preparedplatelet sample is used as-is (resting sample, usually containing morethan 90% resting platelet in healthy subjects), or mixed with a thrombinreceptor-activating peptide (TRAP) to force platelet activation andgenerate a calibration sample with a high percentage (80-90%) ofactivated platelets (activated sample). In some embodiments, the knownparticles 540 are added to the sample, for example at a knownconcentration. In some embodiments, one or more sample preparation stepsare performed in the apparatus, as described below with reference tostep 1517, and the preparation performed in step 1503, if any, iscomplimentary to preparation performed in the apparatus.

In step 1510, the prepared sample is processed in the apparatus, such asapparatus 500 described above with reference to FIG. 5A. One or moresteps performed in the apparatus are described in more detail below.

In step 1590, the condition of the subject from whom the sample wascollected, or a calibration result, is determined based on theproperties derived by the apparatus. For example, the apparatus isdetermined to be calibrated, or the patient is determined to have normalor abnormally elevated numbers of activated platelets. In someembodiments, the apparatus performs other ancillary analyses that allowa more comprehensive determination in step 1590. For example, in someembodiments, based on the presence of another marker together with theresults of the assessment of platelet population, it is determined thata patient from whom the sample was drawn is at increased cardiovascularhealth risk. In another example, changes in proportions of activatedplatelets before and after taking a specific may reflect the efficacy ofthis drug, or on the contrary the resistance of the subject to thisdrug.

In an illustrated embodiment, step 1510 includes one or more of steps1511 through 1530. In step 1511, the sample is introduced at an inputport, such as sample input port 504. For example a pipette containingthe prepared blood sample is emptied into a chamber in the apparatusconnected to the input port 504. In step 1513 one or more sheath fluids(such as immiscible sheath fluids in a preferred embodiment) withdifferent electromagnetic properties from the sample fluid areintroduced at two sheath input ports, such as sheath fluid input ports506 a and 506 b. For example, a reservoir connected to the input ports506 a, 506 b, is filled with mineral oil that has higher electricalimpedance and lower dielectric than the prepared blood sample.

In step 1515, the fluids at the input ports are subjected to pressuresto induce stable fluid flow of sample fluid through the centralmicrochannel. For example, the sample fluid chamber and the reservoir ofsheath fluids are pressurized, respectively, by separate diaphragmsactuated by a pressure source, such as a hydraulic fluid pressurized bya piston or a column of water or other fluid. In another embodiment, thepressurization may be effected by application of a regulated airpressure directly to each fluid. In some embodiments, as describedabove, stability is provided in part by dynamically adjusting thepressure applied to the sample fluid chamber or reservoir of sheathfluid, or both, based on observed core properties, such as width,detected during step 1530, described below. As a result of thepressurization, the sample fluid flows as a core flow between two sheathflows through a microchannel to the exit port, e.g., throughmicrochannels 502 toward exit port 508.

In step 1517, any on-chip conditioning is performed, e.g., as the sampleflows through the sample conditioning components 530. For example,filtering, buffering, diluting and platelet activation, alone or in somecombination, is performed on the sample passing through the sampleconditioning components 530. In some embodiments, the conditioningcomponents 530 add known particles 540 at a known rate into themicrochannel carrying the sample.

In step 1519, the conditioned sample is caused to be sheathed by thesheath fluid. For example, the microchannels 502 carrying sheath fluidmeet the microchannel carrying the conditioned sample at a junction.

In step 1521, particles in the sample are driven toward the center ofthe core flow, e.g., using dielectrophoresis induced by thedielectrophoresis components 550.

In step 1523, the electromagnetic properties are detected in the coreflow, e.g., using core fluid sensors 560. The detection is concentratedin the core flow by the difference in the electromagnetic properties ofthe sample and sheath fluids. For example, optical transmission orelectrical impedance is measured, with greater sensitivity toperturbations that occur in the core flow, as described above.

In step 1530, properties of one or more particles in the sample fluidare derived based on the detected electromagnetic properties. Forexample, temporal or spatial variations in the electromagneticproperties of the core flow are used to characterize particles carriedby the core flow. Variations in the properties of known particles 540are used to determine variations in the core flow, such as core width orcore fluid average properties, and used, in some embodiments, todynamically adjust the pressures on the sample chamber or sheath fluidreservoir, or both, in step 1515. Such dynamic adjustments can beachieved by ultra-rapid pressure-based flow controller such the FluigentMicrofluidic Flow Control System (FASTAB Technology, Fluigent, Paris,France). For example, the percentage of activated platelets isdetermined based on the detected electrical impedance properties of anumber (e.g., at least 50) of platelets.

In some embodiments, step 1530 includes determining properties of thecore flow itself, such as rate or width or both. If the impedance of thecore flow is directly measured with a just one pair of electrode, thenthe core width and average properties can also be detected. This is anadvantageous mode of sensing the core properties in the absence ofparticles. Also, this determination can be done anytime—not only when acalibration particle passes between the electrode, and, thus, canprovide the advantage of a faster real-time control of the core flowproperties.

In some embodiments, step 1530 includes one or more of steps 1531 to1551 of FIG. 15B. In step 1531, the magnitude of the impedance variationis computed at one or more electromagnetic frequencies. For example,magnitude=√(X²+Y²), where X is the in-phase component of the signal andY is the out-of-phase component. Baseline wander (due to slow flowfluctuation for instance) is removed from one or more electromagneticfrequencies, e.g., using a Savitsky-Golay filter. In an experimentalembodiment, the slow variations (baseline wander) are extracted with aSavitzky-Golay filter of low order (e.g., 3), and a frame size longerthan the slowest particle events (e.g., 70 ms, or 501 samples at 7.2kilosamples per second, ksps, where 1 ksps=10³ samples per second).These baseline wanders are then subtracted from the original magnitudesignals for each frequency to leave only fast, particle-related events.

In step 1533, wavelet decomposition is performed at one or moreelectromagnetic frequencies of the measured impedance. Due to theproperties of wavelet transforms, wavelet-based algorithms areinherently well-suited for the detection of transient events (see, forexample, S. Mallat, A Wavelet Tour of Signal Processing, 2 ed. SanDiego, Calif.: Academic, 1999, the entire contents of which are herebyincorporated by reference as if set forth herein, except so far as theterminology is inconsistent with that used here). The algorithm usedtypically two levels of wavelet decomposition to increase robustness. Inone embodiment, a quadratic spline wavelet (see, for example, C. Li, C.X. Zheng, and C. F. Tai, Detection of ECG characteristic points usingwavelet transforms, IEEE Trans. Biomed. Eng., vol. 42, no. 1, pp. 21-28,January 1995, the entire contents of which are hereby incorporated byreference as if set forth herein, except so far as the terminology isinconsistent with that used here) is used to perform a dyadic wavelettransform of the impedance magnitude up to scale 5. Scales 4 and 5 werefound typically to have the most frequency overlap with particle events.Triphasic patterns in the wavelet decomposition, characteristic of thebiphasic impedance change measured by the differential measurementconfiguration, are then searched in scales 4 and 5, typically. Thissearch is done by finding peaks above a defined threshold (set manuallyor calculated from an estimate of the signal noise), and interpretingthe timings of these peaks (two maximum close to a minimum are qualifiedas an event). Events detected on each scale are then pooled to increasethe robustness of the particle detector to noise and variability inevent shapes, and flexibility to various size and types of particles.

In step 1535, the presence of a particle is determined using theWT-based algorithm described above based only on the magnitude signal atthe lowest frequency (found to be the most sensitive to the passage ofparticles).

In step 1537, the real and imaginary part of the impedance variation ateach of one or more electromagnetic frequencies is determined byextraction of peak amplitudes in in-phase components (X) andout-of-phase components (Y) for each frequency.

In step 1538, the magnitudes and phases are then derived. In someembodiments, only amplitude is used, because experiments showed thatphase was mostly redundant. The particle or platelet is thencharacterized by a vector of the relative real components, imaginarycomponents, magnitudes or phases or any combination of one or moreelectromagnetic frequencies and zero or more derived quantities, such asopacity features (ratio of impedances at two frequencies).

In step 1539, amplitudes so detected are normalized by internalcontrols, such as the measured impedances and derived parameters of theknown particles 540. For example, because of the remaining variabilityof the fluidics (core size, flow, centering in channel, sample fluidvariations among samples, among others), the best results were obtainedwhen all platelet parameters (impedances and derived parameters) weredivided by the averaged values measured over the duration of the sampleanalysis for 10-mm polystyrene beads added to the solution as aninternal control. In other embodiments, the normalization is performedusing the value of the nearest calibration bead in the sample stream, orinterpolated between two neighboring calibration beads. Beads can beidentified in the data sets based on a simple magnitude thresholdreflecting large size difference between the beads used as internalcontrols and target particles. In other embodiments, other parametersare used to identify the beads.

In step 1541, the process diverges based on whether calibration orsample analysis is being performed. If calibration, the next step is1543. If the system is already calibrated and is being used to detectproperties of particles in an unknown sample, such as platelets, thenthe next step is 1551.

In step 1543, the vectors associated with known values of the particleproperties are analyzed by cluster analysis to determine how to classifysuch particles, e.g., by defining the center and envelopes of vectorsthat represent each class. In some embodiment, the classes aresufficiently separated that a simple histogram analysis can separate andquantify the various populations, such as between 10 μm polystyrenebeads and 5 μm polyamide beads, or platelet and red blood cells (asdescribed below with reference to FIG. 20B). For populations exhibitingmore subtle differences in impedances (e.g., activated vs. restingplatelet, FIGS. 17A and 17B), statistical approaches are employed. Invarious embodiments, a principal component and discriminant analysiswere used. For example, discriminant analysis is performed, whichdifferentiates between activated and resting platelets within eachcalibration sample. Measurements at multiple frequencies are taken fromtwo samples—one containing resting platelets and 10-μm polystyrenebeads, the other containing activated platelets and the same 10-μmpolystyrene beads. Each sample is typically composed of more than 90% ofthe dominant species—resting or activated. First, data from beads areextracted from the data sets (based on simple magnitude thresholdreflecting large size difference between the beads and platelets). Beadsparameters are averaged over all detected beads and used to normalizeall platelets parameters. Then a discriminant analysis is performed onthe pooled platelet parameters, with all the platelets coming from theresting sample labeled ‘resting’, and all platelets coming from theactivated sample labeled ‘activated’. The discriminant analysis projectsthe data sets onto canonical vectors that have the most discriminatingeffect on the dataset. These vectors, and the associated classcentroids, can then be used to re-classify each sample, or applied tonew samples as in step 1551.

In step 1551, the probability of an unknown particle to belong to theresting platelet or activated platelet class is determined based onpreviously-established classification vectors. Measurement of manyplatelets will thus characterize the population, and lead to a numberreflective of the degree of platelet activation of the sample(percentage of activated platelet). Alternatively, a score based on theunknown platelet's parameter projections on these calibration vectorscan be used to provide a degree of activation for each platelet, ratherthan a bimodal classification.

In order to validate the dielectric hydrodynamic focusing concept,experiments with particles (polystyrene particles with a 10 μm diameter)were performed. These experiments enabled the quantification of theperformance improvement brought by the proposed approach, Thesetechniques were shown to be capable of simple size discrimination basedon the impedance of a particle at a single electromagnetic frequency(1.2 MHz) in which 10 micron particles are polystyrene beads, and 5micron particles are nylon beads from a blood-mimicking solution(Supertech CIRS 046).

Important results include the large improvement brought about by thetwo-phase flow in both sensitivity and signal-to-noise ratio (SNR)compared to standard aqueous hydrodynamic focusing. In the case ofaqueous hydrodynamic focusing, the increase in SNR expected from thereduction of the core is negated somewhat by the diffusion effects,increasing relatively at smaller core width. In contrast, two-phasefocusing gets the full benefits from the narrow core, since diffusion issubstantively non-existent. Average detection signal, ΔI/I_(baseline),was determined for the passage of single 10-micron polystyrene beads ina phosphate buffered saline (PBS) solution with a range of core widthsfrom 27 to 145 microns (all aqueous) and for a two-phase flow (oilsheath, 33 micron core width). For a fixed excitation voltage of 400milliVolts, root mean square (mVrms, 1 mV=10⁻³ volts), a detectionsignal of 1.4±0.6% is measured for a 27-micron aqueous core, whereas thetwo-phase system has a signal of 5.1±0.5% for averages of 21measurements. The signal-to-noise ratio for the various core sizesshowed no significant dependence on core size for the aqueous cores. Forthe two-phase system, a 23 dB improvement compared to the smallestaqueous core is observed.

Using the core fluid impedance sensors FIG. 5B, traces (real part ofimpedance, after baseline-wander removal) of 10 μm polystyrene particlesin oil/water flow were plotted. FIGS. 16A and 16B are graphs thatillustrate example detection of particles in a core flow, according toan embodiment. FIG. 16A depicts a typical trace of these particles. Thehorizontal axis 1602 is time in seconds; and, the vertical axis isimpedance difference between successive electrodes as outputted in voltsby the measuring instrumentation (lock-in amplifier). The trace 1610 isnear zero when there is no particle between either pair of electrodes,and the trace 1610 spikes both negatively and positively as a particlepasses the core fluid sensors (differential configuration). FIG. 16Bdepicts a close-up of individual signals from individual particles, froma separate experiment. The horizontal axis 1622 is an expanded time axisin milliseconds; the vertical axis 1624 represents the impedance changeas outputted in volts by the measuring instrumentation. As shown in FIG.16B, each peak includes a decrease in voltage as the particle passesbetween the first pair of electrodes followed by an increase as theparticle passes between the second pair of electrodes.

4. DETERMINING PLATELET ACTIVATION STATE

According to an example embodiment, the techniques are applied todetermine the number or fraction of platelets in an active state. Theseembodiments address a clinically important application related to themeasurement of platelet activation in blood, and embodies severalaspects of the invention, including measurement of micron-size,shear-sensitive cells.

FIGS. 9A and 9B are micrographs that illustrate a resting platelet andan activated platelet, respectively. FIG. 9A is magnified 10,000 timesand 9B only 5000 times. These micrographs reveal the marked differencein size and morphology that characterizes activation. Blood plateletsare anuclear and discoid, 1.5 to 3.0 μm in diameter. The human body hasa limited reserve of platelets which, therefore, can be depletedrapidly. Platelet activation has been linked to the presence of disease,and therefore is an important disease indicator.

Most previously available tests characterize platelet function as aconsequence of challenge by a chemical or physical agonist (withvariable predictivity) without direct measure of the endogenous plateletstates in blood circulation. Direct indication of the endogenousactivation state is currently only available through the use of flowcytometry, primarily by measuring the presence of certain proteins andreceptors known to be expressed (e.g., CD62p, also known as P-selectin)or known to change conformation (e.g. GPIIb/IIIa) upon activation(Michelson 2000). These methods involve labeling the relevant proteinswith one or more fluorophores or other labels for optical detection.These methods are very sensitive, quantitative, and provide informationon individual platelets. They are, however, expensive, time-consuming,and mostly found in specialized or central laboratories.

One of the main challenges with applying the simpler, cheaper and fastertechniques of impedance spectroscopy to platelets is the small size ofthe platelets. Small platelets will generate a very weak impedancesignal if the channel in which they flow is too large. Reducing channelsize, to achieve a signal to noise ratio (SNR) that is acceptable formeasurements, generates shear that is not suitable for platelets,stressing them sufficiently to damage them or change their activationstate.

In an illustrated embodiment, integrated fluid flow and electricalfinite-element modeling (FEM) are used to properly dimension channelsand flow rates to limit shear stress, while achieving optimalsignal-to-noise ratios in the impedance spectrum measurement. Forexample, the COMSOL Multiphysics software package for Matlab® is used toperform integrated fluidic and electrical finite-element modelingsimulations. High-resolution spectra over a large frequency range arebuilt frequency-by-frequency, averaging multiple platelets (n=200-500)for each frequency. Mixed populations of activated and non-activatedplatelets are delineated statistically. Indeed, it is not practical toobtain a population of 100% non-activated or 100% activated platelets asreference, for various practical reasons, such as residual activationlevel and aggregation. Instead, samples with varying levels ofactivation are used and the distributions of impedances is correlatedwith these levels. This enables the association of specific impedancedistributions with activation levels for each individual frequency.

For example, in an example embodiment, measurements are used tocharacterize the impedance at 20-30 frequencies, over a frequency rangeof 100 Hz-50 MHz. This range should cover three domains of interest: lowfrequencies, where the platelet is basically non-conductive (signalproportional to size), medium frequencies, where the signal is sensitiveto membrane properties (β dispersion), and high frequencies, where thesignal is related to the cytosol structure and properties. It should benoted that while the lower limit of 100 Hz might be practicallydifficult to achieve (electrode polarization, measurement time), theinformation between 100 Hz and 1-10 kHz is not expected to be critical.Analysis of the high-resolution spectra allows determination of a subsetof frequencies having the best discriminating power. The measurementsetup allows simultaneous probing of these frequencies, allowingreal-time characterization of individual platelets.

FIG. 10 is a graph 1000 of simulated data of the impedance spectrum of amixed population of activated and resting platelets, according to anembodiment. The horizontal 1002 axis is frequency in arbitrary units andthe vertical axis 1004 is simulated impedance in arbitrary units. FIG.10 shows a simulated spectrum of impedance measurements 1008 atdifferent AC frequencies. FIGS. 11A through 11F are graphs of simulatedhistogram data of the impedance at various frequencies for plateletpopulations activated to various degree with adenosine diphosphate(ADP), according to an embodiment. Each horizontal axis is 1102impedance in relative units; and each vertical axis 1104 is count, inrelative units. In this simulated case, spectra are clearly differentbetween activated and resting platelets. The histograms on the left(FIG. 11A, FIG. 11C and FIG. 11E) show the relative contributions ofactivation states for a low degree of activation at three discrete ACfrequencies (f1, f2, f3). The histograms on the right (FIG. 11B, FIG.11D and FIG. 11F) show the relative contributions of activation statesas the degree of activation is chemically increased with adenosinediphosphate (ADP) for the same three AC frequencies.

FIG. 12 is a diagram that illustrates electromagnetic measurementapparatus for multiple frequencies, according to an embodiment. This isa particular embodiment of the core fluid sensors 560. This arrangementincludes multiple frequency excitation 1220, with corresponding multiplefrequency discriminators using network analyzer modules 1224, and a fastFourier Transform (FFT) analyzer module 1226. Digital output describingdifferential impedance at multiple frequencies is sent to the computer426. The data are plotted as in FIG. 10 to select discriminatingfrequencies f1, f2, and f3 and generate the histograms of FIGS. 11Athrough 11B. The model includes frequency dependent values of resistanceor capacitance in the membrane 1210 (Rm,Cm), cytoplasm 1212 (Rc),particle 1214 (Rp) and fluid 1216 (Rs) as well as the channel 414 Rc.Resistance of the bridge circuit 1218 Rb is also considered.

In some embodiments, the histograms serve as calibration curves todetermine the activation state of a population of platelets in a samplemeasured using the techniques described herein. A sample with plateletpopulation in a low activation state (corresponding to the absence ofdisease or blood vessel injury) will better match the histograms on theleft side. Conversely, a sample with platelet population in a highactivation state (corresponding to the presence of disease or bloodvessel injury) will better match the histograms on the right side.

Actual experiments with platelets indicated the differences betweenpopulations of activated and non-activated (resting) platelets is moresubtle than depicted in FIG. 11A through 11F. Subsequent to theacquisition of the multi-frequency impedance analyzer, up to eightfrequencies could simultaneously be acquired. A dynamic range andresolution large enough to capture the main transition frequenciesexpected (α and β relaxation) was provided by the instrument.Experiments aiming at increasing the frequency resolution did not appearto add discrimination power.

Most characterizations were done on Platelet-Rich Plasma re-suspended ina citrated tyrode solution with BSA and apyrase to limit activation andaggregation. Activation was performed with a thrombin-receptoractivating peptide (TRAP). Gold-standard activation measurement wasperformed by flow cytometry, using CD61 (platelet marker) and CD62P(P-selectin, marker of activation) antibodies. Non-treated samples wereconsidered ‘resting’. At the dilution and flow rate used, a fewthousands platelets were typically measured in one minute.

The characterization of impedance spectra for resting platelets(non-treated, around 5% activated platelets as confirmed by flowcytometry) and TRAP-activated (around 85% activated platelets asconfirmed by flow cytometry) did not immediately reveal significantdifferences, as expected from the subtle changes in electricalproperties upon activation. For example, FIGS. 17A and 17B are graphs1700 and 1720, respectively, which illustrate example frequency responseof resting and activated platelets, according to an embodiment. Thehorizontal axis 1712 represents electromagnetic frequency in MegaHertz(MHz, 1 MHz=10⁶ hertz). The vertical axes 1704 and 1724 representimpedance difference (X representing in-phase/real, and Y representing90°-phase/imaginary, respectively). Traces 1712 and 1732 indicatenon-treated (resting) platelet samples in the two graphs, respectively;and traces 1714 and 1734 indicate TRAP-activated platelets in the twographs, respectively. No significant difference between resting andactivated platelets is noticed. Data is normalized to internal control(10 micron polystyrene particles). The larger variability in X at 6.04MHz appears to be due to the low impedance of the internal control atthis frequency.

Even so, classification techniques are able to discriminate betweenactivated and resting platelets. FIG. 18 is a graph 1800 thatillustrates example classification of resting and activated platelets,according to an embodiment. The horizontal axis 1802 represents theamplitude of one canonical vector (also called a projection) inarbitrary units; and, the vertical axis 1804 represents the amplitude ofa different, second canonical vector in arbitrary units. The dark datapoints 1812 come from a platelet sample that was activated with TRAP(thrombin receptor agonist); while the gray data points 1814 are from asample of resting platelets. The software uses the different dataparameters derived from the impedance spectroscopy and performs adiscriminant analysis that differentiates between the activated andresting platelets within each sample. In the plot of the first twoprojections (that account for the two highest fractions of the totalvariance), activated platelets are grouped to the left while restingplatelets are grouped to the right. The ellipses 1822 and 1824 of theactivated and resting platelets, respectively, represent 50% of thepopulation within each species.

These results led to other embodiments that consider more advancedstatistical analysis on both raw features (in-phase/out-of-phase partsat various frequencies), and also opacity features (ratio of impedancesat two frequencies). Because of the remaining variability of thefluidics (core size, flow, centering in channel, among others), morefavorable results were obtained when the platelet data were normalizedto polystyrene beads added to the solution as an internal control. Theseresults are shown in FIG. 19, along with calibration data from FACSmeasurements.

FIG. 19 is a graph that illustrates example agreement of degree ofplatelet activation with a standard, according to an embodiment. Thehorizontal axis 1902 indicates different groups of samples and thevertical axis 1904 indicates the number of activated platelets in thegroup as a percentage. Comparison of percentages of activation derivedfrom microchannel impedance data, EIS 1912 a 1912 b (N=24) andconventional FACS analysis 1910 a 1910 b in two types of samples—resting(non-activated platelets) and activated (TRAP-activated platelets),respectively. Comparable trends demonstrate the ability of EIS toquantify the level of activation. Note that exact match between FACS andEIS was not expected due to the different features measured. EIS Subset1914 a and 1914 b (N=5) represent a subset of data selected for theirlow variability in the internal control (polystyrene beads), an overallindicator of the stability of the fluidics. Stability of the internalcontrol was assessed by the lack of discrimination of the controlparticles in the various samples based on the canonical vectors trainedon the platelets.

One can see that the percentages of activated platelets calculated fromimpedance data are comparable to those obtained by the standard methodof flow cytometry. It is important to note however that they should notbe expected to be exactly equal, as both methods are measuring differentindicators of activation. Because activation is more a graded state thanan on/off state, different indicators would result in similar, butslightly different outcomes. This also shows that by down-selecting toonly those datasets where the internal control was highly stable (anindication of stable measurement conditions), results could be furtherimproved. This indicates that further refinement of the microfluidicsystem will likely increase the performance of the method (also calledan assay in this context).

It was also verified that the device and measurement process were notleading to auto-activation of the platelet (through reactions withsurfaces or shear). Flow cytometry measurements of resting plateletsbefore and after measurements (collected at the output of the device)showed no significant difference (6.1% before, 5.9% after), in effectvalidating the low-shear design.

This project has led to the successful development of amicrofluidic-based impedance flow cytometer, and the demonstration of anovel approach to maintain the signal-to-noise ratio of small channelswhile keeping large physical dimensions—two normally opposed parameters,both desirable simultaneously for the analysis of platelets withoutinducing activation. Several fluidic issues were investigated andfeasibility demonstrated. Strong evidence supports the possibility ofquantifying the levels of platelet activation in a sample by electricalimpedance spectroscopy in a microfluidic device. It is anticipated thatthese techniques provide a means for the detection of disease andefficacy of disease treatment, e.g., to monitor platelet interactionswith anti-platelet drugs such as clopidogrel or aspirin.

5. CHARACTERIZATION OF RED BLOOD CELLS (RBCS) AND LEUKOCYTES

Ideally, measurements should be performed on whole blood, allowinginformation on other blood constituents to be measured in addition toplatelet data (e.g. red and white blood cell counts). The samplepreparation would also be greatly simplified. Undiluted whole blood,however, contains so many cells that it is difficult to obtain signalsfrom single cells. Alternatives would be to either dilute the blood orto selectively lyse the red blood cells as they are the most prevalentcell type in the blood. Diluting results in a sample that is easier tohandle and the ratio between platelets and other blood cells will remainunchanged if the dilution is properly performed while a large number ofcells will have to be analyzed in order to get statistically relevantdata on the platelets. Lysing of the red blood cells will create ghostcells and membrane residues that, without careful washing of the sample,may interfere with the measurements.

However, limited experiments with samples prepared by differentialcentrifugation (reducing predominantly the concentration of RBCs) showedthat it was possible to get information on both cell types, and thatdiscrimination between both might be achievable by simple sizediscrimination (using typically the in-phase component of the impedanceat low frequency).

FIG. 20A and FIG. 20B are graphs that illustrate example detection ofplatelets and red blood cells in a blood sample, according to anembodiment. Depicted is a partly centrifuged whole blood sample. In FIG.20A, in graph 2000, the horizontal axis 2002 is time in samples perseconds (sampling rate: 7.2 ksps) and the vertical axis 2004 isimpedance signal in volts, as outputted by the measuringinstrumentation. Trace 2010 is a typical impedance signal from a mixedpopulation of platelet and RBCs. The trace 2010 is a raw signal forin-phase component of impedance (X), showing the different signature ofred blood cells (RBCs, also called erythrocytes) and platelets. The RBCscause the large peaks; and the platelets cause the smaller peaks.

FIG. 20B depicts a histogram 2020 of size distributions. The horizontalaxis 2022 represents the X impedance value at low frequency(size-dependant); and the vertical axis 2024 indicates the number ofpeaks (count). The histogram 2030 shows the small distribution ofplatelets (low amplitudes) and the overwhelming distribution of RBCs,even in a partially-centrifuged sample. On the basis of these resultswith RBCs, discrimination of large leukocytes from small platelets basedon size appears feasible.

6. COMPUTATIONAL HARDWARE OVERVIEW

FIG. 13 is a block diagram that illustrates a computer system 1300 uponwhich an embodiment of the invention may be implemented. Computer system1300 includes a communication mechanism such as a bus 1310 for passinginformation between other internal and external components of thecomputer system 1300. Information is represented as physical signals ofa measurable phenomenon, typically electric voltages, but including, inother embodiments, such phenomena as magnetic, electromagnetic,pressure, chemical, molecular atomic and quantum interactions. Forexample, north and south magnetic fields, or a zero and non-zeroelectric voltage, represent two states (0, 1) of a binary digit (bit).).Other phenomena can represent digits of a higher base. A superpositionof multiple simultaneous quantum states before measurement represents aquantum bit (qubit). A sequence of one or more digits constitutesdigital data that is used to represent a number or code for a character.In some embodiments, information called analog data is represented by anear continuum of measurable values within a particular range. Computersystem 1300, or a portion thereof, constitutes a means for performingone or more steps of one or more methods described herein.

A sequence of binary digits constitutes digital data that is used torepresent a number or code for a character. A bus 1310 includes manyparallel conductors of information so that information is transferredquickly among devices coupled to the bus 1310. One or more processors1302 for processing information are coupled with the bus 1310. Aprocessor 1302 performs a set of operations on information. The set ofoperations include bringing information in from the bus 1310 and placinginformation on the bus 1310. The set of operations also typicallyinclude comparing two or more units of information, shifting positionsof units of information, and combining two or more units of information,such as by addition or multiplication. A sequence of operations to beexecuted by the processor 1302 constitutes computer instructions.

Computer system 1300 also includes a memory 1304 coupled to bus 1310.The memory 1304, such as a random access memory (RAM) or other dynamicstorage device, stores information including computer instructions.Dynamic memory allows information stored therein to be changed by thecomputer system 1300. RAM allows a unit of information stored at alocation called a memory address to be stored and retrievedindependently of information at neighboring addresses. The memory 1304is also used by the processor 1302 to store temporary values duringexecution of computer instructions. The computer system 1300 alsoincludes a read only memory (ROM) 1306 or other static storage devicecoupled to the bus 1310 for storing static information, includinginstructions, that is not changed by the computer system 1300. Alsocoupled to bus 1310 is a non-volatile (persistent) storage device 1308,such as a magnetic disk or optical disk, for storing information,including instructions, that persists even when the computer system 1300is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 1310 for useby the processor from an external input device 1312, such as a keyboardcontaining alphanumeric keys operated by a human user, or a sensor. Asensor detects conditions in its vicinity and transforms thosedetections into signals compatible with the signals used to representinformation in computer system 1300. Other external devices coupled tobus 1310, used primarily for interacting with humans, include a displaydevice 1314, such as a cathode ray tube (CRT) or a liquid crystaldisplay (LCD), for presenting images, and a pointing device 1316, suchas a mouse or a trackball or cursor direction keys, for controlling aposition of a small cursor image presented on the display 1314 andissuing commands associated with graphical elements presented on thedisplay 1314.

In the illustrated embodiment, special purpose hardware, such as anapplication specific integrated circuit (IC) 1320, is coupled to bus1310. The special purpose hardware is configured to perform operationsnot performed by processor 1302 quickly enough for special purposes.Examples of application specific ICs include graphics accelerator cardsfor generating images for display 1314, cryptographic boards forencrypting and decrypting messages sent over a network, speechrecognition, and interfaces to special external devices, such as roboticarms and medical scanning equipment that repeatedly perform some complexsequence of operations that are more efficiently implemented inhardware.

Computer system 1300 also includes one or more instances of acommunications interface 1370 coupled to bus 1310. Communicationinterface 1370 provides a two-way communication coupling to a variety ofexternal devices that operate with their own processors, such asprinters, scanners and external disks. In general the coupling is with anetwork link 1378 that is connected to a local network 1380 to which avariety of external devices with their own processors are connected. Forexample, communication interface 1370 may be a parallel port or a serialport or a universal serial bus (USB) port on a personal computer. Insome embodiments, communications interface 1370 is an integratedservices digital network (ISDN) card or a digital subscriber line (DSL)card or a telephone modem that provides an information communicationconnection to a corresponding type of telephone line. In someembodiments, a communication interface 1370 is a cable modem thatconverts signals on bus 1310 into signals for a communication connectionover a coaxial cable or into optical signals for a communicationconnection over a fiber optic cable. As another example, communicationsinterface 1370 may be a local area network (LAN) card to provide a datacommunication connection to a compatible LAN, such as Ethernet. Wirelesslinks may also be implemented. Carrier waves, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared wavestravel through space without wires or cables. Signals include man-madevariations in amplitude, frequency, phase, polarization or otherphysical properties of carrier waves. For wireless links, thecommunications interface 1370 sends and receives electrical, acoustic orelectromagnetic signals, including infrared and optical signals, thatcarry information streams, such as digital data.

The term computer-readable medium is used herein to refer to any mediumthat participates in providing information to processor 1302, includinginstructions for execution. Such a medium may take many forms,including, but not limited to, non-volatile media, volatile media andtransmission media. Non-volatile media include, for example, optical ormagnetic disks, such as storage device 1308. Volatile media include, forexample, dynamic memory 1304. Transmission media include, for example,coaxial cables, copper wire, fiber optic cables, and waves that travelthrough space without wires or cables, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared waves. Theterm computer-readable storage medium is used herein to refer to anymedium that participates in providing information to processor 1302,except for transmission media.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, a hard disk, a magnetic tape, or any othermagnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD)or any other optical medium, punch cards, paper tape, or any otherphysical medium with patterns of holes, a RAM, a programmable ROM(PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memorychip or cartridge, a carrier wave, or any other medium from which acomputer can read.

Logic encoded in one or more tangible media includes one or both ofprocessor instructions on a computer-readable storage media and specialpurpose hardware, such as ASIC 1320.

Network link 1378 typically provides information communication throughone or more networks to other devices that use or process theinformation. For example, network link 1378 may provide a connectionthrough local network 1380 to a host computer 1382 or to equipment 1384operated by an Internet Service Provider (ISP). ISP equipment 1384 inturn provides data communication services through the public, world-widepacket-switching communication network of networks now commonly referredto as the Internet 1390. A computer called a server 1392 connected tothe Internet provides a service in response to information received overthe Internet. For example, server 1392 provides information representingvideo data for presentation at display 1314.

The invention is related to the use of computer system 1300 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 1300 in response to processor 1302 executing one or moresequences of one or more instructions contained in memory 1304. Suchinstructions, also called software and program code, may be read intomemory 1304 from another computer-readable medium such as storage device1308. Execution of the sequences of instructions contained in memory1304 causes processor 1302 to perform the method steps described herein.In alternative embodiments, hardware, such as application specificintegrated circuit 1320, may be used in place of or in combination withsoftware to implement the invention. Thus, embodiments of the inventionare not limited to any specific combination of hardware and software.

The signals transmitted over network link 1378 and other networksthrough communications interface 1370, carry information to and fromcomputer system 1300. Computer system 1300 can send and receiveinformation, including program code, through the networks 1380, 1390among others, through network link 1378 and communications interface1370. In an example using the Internet 1390, a server 1392 transmitsprogram code for a particular application, requested by a message sentfrom computer 1300, through Internet 1390, ISP equipment 1384, localnetwork 1380 and communications interface 1370. The received code may beexecuted by processor 1302 as it is received, or may be stored instorage device 1308 or other non-volatile storage for later execution,or both. In this manner, computer system 1300 may obtain applicationprogram code in the form of a signal on a carrier wave.

Various forms of computer readable media may be involved in carrying oneor more sequence of instructions or data or both to processor 1302 forexecution. For example, instructions and data may initially be carriedon a magnetic disk of a remote computer such as host 1382. The remotecomputer loads the instructions and data into its dynamic memory andsends the instructions and data over a telephone line using a modem. Amodem local to the computer system 1300 receives the instructions anddata on a telephone line and uses an infra-red transmitter to convertthe instructions and data to a signal on an infra-red a carrier waveserving as the network link 1378. An infrared detector serving ascommunications interface 1370 receives the instructions and data carriedin the infrared signal and places information representing theinstructions and data onto bus 1310. Bus 1310 carries the information tomemory 1304 from which processor 1302 retrieves and executes theinstructions using some of the data sent with the instructions. Theinstructions and data received in memory 1304 may optionally be storedon storage device 1308, either before or after execution by theprocessor 1302.

FIG. 14 illustrates a chip set 1400 upon which an embodiment of theinvention may be implemented. Chip set 1400 is programmed to perform oneor more steps of a method described herein and includes, for instance,the processor and memory components described with respect to FIG. 13incorporated in one or more physical packages (e.g., chips). By way ofexample, a physical package includes an arrangement of one or morematerials, components, and/or wires on a structural assembly (e.g., abaseboard) to provide one or more characteristics such as physicalstrength, conservation of size, and/or limitation of electricalinteraction. It is contemplated that in certain embodiments the chip setcan be implemented in a single chip. Chip set 1400, or a portionthereof, constitutes a means for performing one or more steps of amethod described herein.

In one embodiment, the chip set 1400 includes a communication mechanismsuch as a bus 1401 for passing information among the components of thechip set 1400. A processor 1403 has connectivity to the bus 1401 toexecute instructions and process information stored in, for example, amemory 1405. The processor 1403 may include one or more processing coreswith each core configured to perform independently. A multi-coreprocessor enables multiprocessing within a single physical package.Examples of a multi-core processor include two, four, eight, or greaternumbers of processing cores. Alternatively or in addition, the processor1403 may include one or more microprocessors configured in tandem viathe bus 1401 to enable independent execution of instructions,pipelining, and multithreading. The processor 1403 may also beaccompanied with one or more specialized components to perform certainprocessing functions and tasks such as one or more digital signalprocessors (DSP) 1407, or one or more application-specific integratedcircuits (ASIC) 1409. A DSP 1407 typically is configured to processreal-world signals (e.g., sound) in real time independently of theprocessor 1403. Similarly, an ASIC 1409 can be configured to performedspecialized functions not easily performed by a general purposedprocessor. Other specialized components to aid in performing theinventive functions described herein include one or more fieldprogrammable gate arrays (FPGA) (not shown), one or more controllers(not shown), or one or more other special-purpose computer chips.

The processor 1403 and accompanying components have connectivity to thememory 1405 via the bus 1401. The memory 1405 includes both dynamicmemory (e.g., RAM, magnetic disk, writable optical disk, etc.) andstatic memory (e.g., ROM, CD-ROM, etc.) for storing executableinstructions that when executed perform one or more steps of a methoddescribed herein. The memory 1405 also stores the data associated withor generated by the execution of one or more steps of the methodsdescribed herein.

7. ALTERATIONS AND MODIFICATIONS

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method comprising: causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is substantively less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes.
 2. A method as recited in claim 1, further comprising determining impedance of a particle in the core fluid.
 3. A method as recited in claim 2, wherein determining impedance of the particle in the core fluid further comprises determining a temporal change in measured impedance in the channel between the first pair of electrodes.
 4. A method as recited in claim 2, wherein determining impedance of the particle in the core fluid further comprises determining a difference between a first determined impedance in the channel at the first pair of electrodes and a second determined impedance at a different second pair of electrodes disposed along the channel separately from the first pair of electrodes.
 5. A method as recited in claim 2, wherein a largest spatial dimension of the particle is much less than a narrowest spatial dimension of the channel.
 6. A method as recited in claim 2, wherein a largest spatial dimension of the particle is not substantively greater than a narrowest spatial dimension of the core fluid.
 7. A method as recited in claim 2 wherein the particle is a platelet.
 8. A method as recited in claim 7, further comprising distinguishing an activated platelet and resting platelet based, at least in part, on impedance determined for the particle.
 9. A method as recited in claim 1, wherein determining impedance in the channel at a first pair of electrodes further comprises determining impedance in the channel at the first pair of electrodes at a plurality of alternating current frequencies.
 10. A method as recited in claim 9, wherein the plurality of alternating current frequencies span a frequency range greater than about 100 megahertz (MHz).
 11. A method as recited in claim 1, wherein causing the core fluid to flow into the channel between two sheath flows further comprises controlling a narrowest spatial dimension of the core by controlling relative pressure or flow rate of a core fluid compared to a corresponding pressure or flow rate of one or more of the sheath fluids.
 12. A method as recited in claim 11, wherein controlling relative pressure or flow rate of the core fluid compared to the corresponding pressure or flow rate of the one or more of the sheath fluids further comprises controlling the relative pressure or flow rate to stabilize a measurement of a property of the core.
 13. A method as recited in claim 12, wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes.
 14. A method as recited in claim 12, wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes when a particle is absent from the channel between the first pair of electrodes.
 15. A method as recited in claim 12, wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes at a particular set of one or more alternating current frequencies.
 16. A method as recited in claim 12, wherein the measurement of the property of the core is the width of the core between the two sheath flows in the channel in a vicinity of the first pair of electrodes.
 17. A method as recited in claim 12, wherein the measurement of the property of the core is based on a measurement of a property of an internal control.
 18. A method as recited in claim 17, wherein the internal control is a plurality of known particle of uniform properties.
 19. A method as recited in claim 1, wherein the core fluid comprises substantively polar molecules and the sheath fluid comprises substantively non-polar molecules.
 20. A method as recited in claim 19, wherein a width of the core between the sheath flows in the channel is controlled, at least in part, by a width of a strip of material with affinity for the polarity of the core fluid in at least one of a top wall or a bottom wall of the channel.
 21. A method as recited in claim 19, wherein a width of the core between the sheath flows in the channel is controlled, at least in part, by topographical features on at least one of a top wall or a bottom wall of the channel, which features extend into the channel.
 22. A method as recited in claim 1, wherein the sheath fluid is mineral oil and the core fluid is an aqueous mixture.
 23. A method as recited in claim 1, wherein the sheath fluid is a fluorocarbon solvent and the core fluid is an aqueous mixture.
 24. A method as recited in claim 1, wherein the sheath fluid is a gas.
 25. A method as recited in claim 24, wherein the gas is air.
 26. A method as recited in claim 1, wherein the method further comprises measuring an optical property in the channel.
 27. A method as recited in claim 1, wherein the core fluid has a higher index of refraction for optical waves than do the one or more sheath fluids.
 28. A method as recited in claim 27, wherein measuring the optical property further comprises directing incident light to produce substantively total internal reflection within a core flow that encompasses the core fluid.
 29. A method as recited in claim 1, further comprising determining impedance in the channel at a plurality of pairs of electrodes disposed separately along the channel and disposed separately from the first pair of electrodes.
 30. A method as recited in claim 2, wherein: the method further comprises introducing a plurality of uniform particles of known impedance value into the core flow as an internal control; and, determining impedance of the particle in the core fluid further comprises normalizing features of measured impedance based, at least in part, on measured value of impedance for at least one particle of the internal control.
 31. A method comprising: causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein an optical index of refraction of a fluid constituting either sheath flow is much less than an optical index of refraction of the core fluid; and measuring an optical property in the channel between an optical source and an optical detector.
 32. A method as recited in claim 31, further comprising determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes.
 33. A method as recited in claim 31, wherein measuring the optical property further comprises directing incident light to produce substantively total internal reflection within a core flow that encompasses the core fluid.
 34. A method as recited in claim 31, further comprising determining a property of a core flow that encompasses the core fluid based on the optical property measured.
 35. A method as recited in claim 31, further comprising determining a property of a particle in a core flow that encompasses the core fluid based on the optical property measured.
 36. An apparatus comprising: means for causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is substantively less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and means for determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes.
 37. A non-transient computer-readable medium carrying one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes an apparatus to: receive first data indicating measurements of impedance in a channel between a first pair of electrodes, wherein a core fluid flows into the channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and determine impedance of a particle in the core fluid based at least in part on the first data.
 38. A computer-readable medium as recited in claim 37, wherein the apparatus is further caused to determine control data that indicates, at least in part, relative pressure of a source of the core fluid compared to pressure of a source of one or more of the sheath fluids.
 39. A computer-readable medium as recited in claim 38, wherein to determine control data further comprises to determine control data to stabilize a value of the first data.
 40. A computer-readable medium as recited in claim 38, wherein: the apparatus is further caused to receive second data that indicates measurements of a property of the core fluid; and to determine control data further comprises to determine control data to stabilize a value of the second data.
 41. A computer-readable medium as recited in claim 40, wherein the second data indicates measurements of width of the core fluid.
 42. An apparatus comprising: means for receiving first data indicating measurements of impedance in a channel between a first pair of electrodes, wherein a core fluid flows into the channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and means for determining impedance of a particle in the core fluid based at least in part on the first data.
 43. A method comprising: causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a value of a first electromagnetic property of a fluid constituting either sheath flow is substantially different from a value of the first electromagnetic property of the core fluid, and wherein flow in the channel is laminar; and measuring a second electromagnetic property in the channel using an electromagnetic signal that is concentrated in the core fluid by a difference in the value of the first electromagnetic property of either sheath flow and the value of the first electromagnetic property of the core fluid. 